1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying a cut sheet in an image forming apparatus such as a printer, a facsimile, a copying machine, a printing device and the like.
2. Related Background Art
In image forming apparatuses such as copying machines, printers, facsimiles and the like, there has widely been used an apparatus for fixing a non-fixed toner image formed on a material to be recorded (for example, a paper sheet such as transfer material, photosensitive paper, electrostatic recording paper and printing paper) in correspondence to image information with a transfer system (indirect system) or a direct system by using an appropriate imaging process mechanism of electrophotographic type, electrostatic type or magnetic recording type.
Further, there has been used an apparatus (of ink jet type, for example) for directly forming an image on a material to be recorded, by using liquid including dye or/and pigment.
As a sheet conveying apparatus for conveying the material to be recorded (referred to as xe2x80x9csheetxe2x80x9d hereinafter) in the above-mentioned apparatuses, there is a sheet conveying apparatus of automatic feeding type in which one cassette corresponding to desired sheet kind or sheet size is selected among a plurality of sheet feeding cassettes each containing sheets and a sheet is fed out one by one from such a sheet feeding cassette in synchronous with an image forming operation.
In such a sheet conveying apparatus, the following sheet detecting means have widely been used as a means for detecting sheet jam or detecting a leading end or a trailing end of the sheet.
A. Lever Type
As shown in FIG. 22A, a leading end of a sheet P conveyed along between guides G falls a lever L down so that an photo-interrupter I is blocked by a flag F opposite end of the lever, thereby detecting the leading end of the sheet. The lever L is biased in a clockwise direction by a spring or a gravity force to the extent that the sheet is not buckled.
B. Sensor of Reflection Type
An infrared ray is emitted toward a sheet, and reflected light or scattered light from the sheet is detected by a light receiving element. On the basis of a light amount received, presence/absence of the sheet is judged.
C. Sensor of Permeable Type
An infrared ray is emitted toward a sheet, and light passed through the sheet is received by a light receiving element. On the basis of a light amount passed, presence/absence of the sheet is detected.
In recent years, digitization has been progressed. For example, the more the number of revolutions of a polygon mirror is increased and drive frequencies of an LED head and an ink jet head are increased the more technical problems are increased, thereby increasing solution cost. Thus, in apparatuses having the same image forming speed, it has been requested that a distance between the sheets continuously conveyed be decreased to improve productivity. Further, regarding energy aspect and endurance aspect of all of parts moved or rotated, it is desirable that the image forming speed be reduced as less as possible.
However, if the distance between the sheets continuously conveyed is reduced to zero ultimately, since the leading and trailing ends of the sheet cannot be detected, control for sheet conveyance cannot be effected. For example, alignment between the sheet and an image position and detection of a jammed sheet cannot be effected.
In the lever type (A), as shown in FIG. 2B, if the trailing end of the sheet does not pass through the distal end of the fallen lever L, since the lever cannot be returned to its original position, at least a sheet-to-sheet distance (La) corresponding to a length of the lever L is required, and a time period of several tends of milliseconds is required for returning the lever L. Regarding this time period, as the sheet conveying speed is increased, loss of the sheet-to-sheet distance becomes greater; for example, when the time period for returning the lever L is 30 milliseconds and the sheet conveying speed is 500 mm/sec, the sheet-to-sheet distance of 15 mm is further required.
In the sensor of reflection type (B) and the sensor of permeable type (C), although the loss of the sheet-to-sheet distance is smaller than in the lever type, the sheet-to-sheet distance of about 5 mm is required for preventing erroneous detection.
The present invention aims to eliminate the above-mentioned conventional drawbacks, and an object of the present invention is to provide a sheet conveying apparatus in which, even when a sheet-to-sheet distance is approached to zero, the sheet can be detected.
According to the present invention, there is provided a sheet conveying apparatus comprising a sheet feeding means for separating and feeding sheets one by one, a convey guide means for guiding the sheet fed out by the sheet feeding means, a spacing means provided on the guide means and adapted to deviate positions of a trailing end of a preceding sheet and a leading end of a succeeding sheet which is fed continuously to the preceding sheet, and a sheet detecting mean for detecting the leading end of the succeeding sheet deviated from the trailing end of the preceding sheet by the spacing means.